In conventional furnaces for heating or heat treatment, heat radiation by a radiant tube of the combustion type has been widely employed. It has been esteemed as highly economical in the operation cost compared to electrical heating, at least in the furnaces in the neighborhood of 800.degree. C. It can not be, however, free from some problems or shortcomings, which have even been growing in recent time. Some of those problems or shortcomings will be mentioned by way of example:
(1) The furnace size has been growing larger aiming at the improvement of its thermal efficiency and productivity, with the result of installing multiple radiant tubes in one furnace, which brings about increasing of the ambient temperature and the combustion noise. PA1 (2) The amount of fuel burnt in one burner has been increased for getting a larger combustion heat at a time for the same purpose as the above, which also brings about increasing of the environmental temperature and the noise in proportion thereto. PA1 (3) The above two matters caused some new problems by bringing about a deterioration of the working environment and a sound nuisance to the regional community. As a countermeasure for preventing the noise nuisance, an attempt to cover the entire factory building with sound absorbing materials was made. A tremendous cost and undue rising of the room temperature doomed it as impracticable. PA1 (4) Another attempt was made, in addition to the above measure, of installing air conditioning equipment in the factory, which was of course too expensive to be practiced. PA1 (5) In the furnaces of the conventional type to be used at a temperature lower than 800.degree. C the effect of air preheating by means of a heat exchanger is far inferior to those used at 1,000.degree. C or higher. The preheated temperature of the air does not exceed 250.degree. C and the rise of thermal efficiency can not be expected to be more than 10 - 13%. Trying to raise this efficiency unreasonably is frustrated by the rising of the back-pressure in the burner, which deteriorates the combustion conditions. It has been generally said that an air preheating does not pay in furnaces operating at less than 800.degree. C.